Even on a night which is too dark for diurnal vision, invisible infrared light is richly provided by the stars. Human vision cannot utilize this infrared night time light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night time scene. Such night vision devices generally include an objective lens which focuses invisible infrared light from the night time scene onto the transparent light-receiving face of an I.sup.2 T. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an I.sup.2 T with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of dynodes, or microchannels, with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern results. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen by another electrostatic field to produce a visible image.
A power supply for the I.sup.2 T provides the electrostatic field potentials referred to above, and also provides a field and current flow to the microchannel plate(s). Conventional night vision devices (i.e., since the 1970's and to the present day) provide automatic brightness control (ABC), and bright source protection (BSP). The former function maintains the brightness of the image provided to the user substantially constant despite changes in the brightness (in infrared and the nearinfrared portion of the spectrum) of the scene being viewed. BSP prevents the I.sup.2 T from being damaged by an excessively high current level in the event that a bright source, such as a flare or fire, comes into the field of view.
The ABC function is accomplished by providing a regulator circuit monitoring the output current from the phosphorescent screen (See FIG. 9). When this current exceeds a certain threshold, the field voltage level across the opposite faces of the microchannel plate(s) is decreased to reduce the gain of the microchannel plate(s), as is graphically depicted in FIG. 10. This reduction of microchannel plate voltage also has the effect of reducing the resolution of the I.sup.2 T. That is, the gain versus voltage function of the I.sup.2 T at lowered MCP voltages results in a matrix pattern from the microchannel plate(s) appearing in the image. This matrix pattern is sometimes referred to as fixed-pattern noise in the image. As a result, in bright-field conditions with the ABC feature of the conventional night vision device operating the conventional night vision device may drastically lose resolution so that the user of the device is no longer able to discern details of the viewed scene which would be discernible were they viewed under darker field conditions in which ABC were not applying.
An additional bright source protection is provided in conventional night vision devices by decreasing the field voltage provided to the photocathode. This happens because the high impedance of the photocathode in combination with a high resistance value circuit element creates a greater voltage drop under the high current conditions caused by a large number of photons incident on the photocathode (with a resulting high number of photoelectrons being provided by the photocathode). The photoelectrons provided by the photocathode represent a current flow increasing in magnitude with increasing light levels in the viewed field, such that the combined impedance of the photocathode and circuit element causes a decrease in the voltage level effective at the photocathode to move these electrons to the microchannel plate(s).
Recalling FIG. 9, it will be noted that this circuit architecture requires the use of two transformers, which are relatively large and heavy components of the circuit. Further, is seen that a typical conventional circuit architecture for a power supply of a night vision device provides a high-value resistor (generally 1-18 G-ohm) to the output of the photocathode voltage multiplier and a clamping circuit consisting of a voltage source and a low-leakage, high-voltage diode. As photocathode current flows through the high-value resistor, the photocathode voltage will decrease linearly until it reaches a voltage equal to the voltage source (plus the high-voltage low-leakage diode voltage drop). See FIG. 11 for a graphical illustration of this BSP voltage relationship at the photocathode. This voltage is commonly referred to as a clamp voltage, and is typically between 30 and 40 volts D.C.
This conventional method of BSP also has a disadvantage of decreased resolution for the I.sup.2 T. The reduced electrostatic field between the photocathode and the microchannel plate(s) input causes a reduced resolution for the tube. That is, photoelectrons liberated from the photocathode are not moved to the microchannel plate(s) as effectively, and may not be liberated to reach the microchannel plate(s) at all. This is because photoelectrons must overcome a surface potential barrier at the photocathode in order to be liberated into free space and be moved by the prevailing electrostatic field to the input of the microchannel plate(s). As the voltage applied to the photocathode decreases, statistically, some photoelectrons will not be able to overcome this surface potential barrier and will not be liberated into free space. The image information represented by these trapped photoelectrons will be lost from the image provided by the I.sup.2 T to the user of the night vision device.